Milling vs. Grinding
A discussion of various machining characteristics in regard to milling and grinding. A dialogue in support of magnetic workholding and abrasive machining. Although this document is from our Magna-Lock USA archives and appears to be many years old, it contains information that many of our customers have found pertinent when considering equipment choices and machining techniques with larger mills and surface grinders.
Uniform Hardness of Material
Milling says : Avoid sudden changes in cross section. Although this is considered no more than ordinary good design practice, it is mentioned here because of its effect on the casting. Thin sections, which cool more quickly, can result in chill spots which are higher hardness than adjacent sections. Such abrupt variations in hardness can have disastrous effects on the cemented carbide cutting edge.
Grinding says : Chill spots in castings are note of concern on a vertical spindle surface grinder. Chill spots can have a considerable variation in hardness from heavy sections in the same casting without causing trouble. Uniform sections and uniform hardness are not part of Abrasive Machining requirements. The grinding wheel has no memory and does not care whether the part is hard or soft. Hard and soft spots all help the grinding wheel to dress itself.
Milling says : Provide enough material for a good machining cut. When the tool must make a scale cut, the effective lift of the cutting edge is reduced. Bear in mind that you do not always reduce machining time when you leave little stock; you may in fact be losing time and money as a result of excessive cutting edge wear. Try to leave enough material to assure that the nose of the tool is always under the scale.
Grinding says : Baloney. Why pay to put on stock, and then turn around and pay to take it off again? Abrasive Machining requires only enough stock to assure that the part can be ground to the required geometry. In some applications, it may not even be necessary to remove all the scale as long as the surface is solid. This offers considerable and definite material savings, plus the savings from not removing stock that is not on the casting.
Milling says : Avoid interrupted cuts. An interrupted cut is one where the tool alternately enters and leaves the workpiece during the cut. Because cast iron has relatively low tensile strength, the tendency of the cutting forces it to break out the edge of the casting as the tool leaves the work. Although the interrupted cut is hard on the tool, a suitable tool material capable of withstanding impact shock loading can be selected. A side effect is the potential production of thermal cracks in the tool material.
Grinding says : Interrupted cuts are exactly what the grinding wheel needs to help in self-dressing. There is no need to baby the wheel, and no concern about edges in the casting. This type of surface allows for easy application of the coolant, either through the wheel or from the outside.
Milling says : Avoid thin-walled sections. Thin walled castings are difficult to deal with. They are often hard to hold in chucks and fixtures. During machining, the thin walls tend to produce high frequency vibration. The result of this vibration can be seen as “chatter marks” on the workpiece and as chipped or broken tools.
Grinding says : Of course, everybody should try to avoid thin-walled sections, but there are times when design characteristics will not allow it. The rigidity of the grinder, using the minimum feed available, can help adapt grinding technique to the part. It is unusual to have to consider vibration of the thin sections. If vibrations do occur, in roughing, it is usually possible to remove any resulting traces of chatter in the sparkout passes.
Milling says : Remember a casting has a draft. This makes the depth of the cut vary when cutting with the draft. Make sure that the machine has enough horsepower and the work holders sufficient strength to handle the greatest depth of cut which will be encountered. This is particularly important when machining to a corner or shoulder. Stalling of the machine could result in considerable damage to the workpiece, the cutting tool, and the machine tool itself.
Grinding says : Draft is not a problem in abrasive machining. A grinder has a table that is constantly rotating, or reciprocating, under a continuous downfeed of the grinding wheel. The “high” point of the work is contacted first with gradual feed into the full surface area. Remember that the grinding wheel diameter is at least 1⁄2 the diameter of a rotary table and cover completes the width of the table and a reciprocating grinding machine.
Use of Coolant
Milling says : Consider proper application of coolant. Although many cast iron machining applications can be performed satisfactorily in the dry condition, properly applied cutting fluids can extend tool life. Cutting fluids are best applied under pressure directly at the tool-chip interface. The use of coolant is particularly dangerous, however, in interrupted cut applications. Because if may promote the formation of heat checks in the edge of the cemented carbide tool.
Grinding says : Too bad that the cemented carbide tool is so sensitive to coolant. Coolants are a must for abrasive machining for many reasons; one of the principal ones being to keep the work cool. Coolants on high-powered vertical spindle surface grinders provide better cutting action and longer wheel life. This is documented. Finally, the coolant absolutely carries away all the chips and swarf.
Make sure that the machine has enough horsepower and the work holders sufficient strength to handle the greatest depth of cut which will be encountered.
Entry and Exit Angles
Milling says : Have the tool enter and leave the work gradually. This is a machining technique that can aid in avoiding heat checks and chipping out of the casting. The use of rounded corners and gradual sections changes in the design of the casting can help in promoting this technique. Tool geometry can help in achieving this objective. Use of a high lead angle helps to avoid abrupt entry and exit.
Grinding says : There is total wheel contact in abrasive machining on a vertical spindle grinder, and line contact on any type of grinder. There is nothing like cutter vibration or tooth-load. The direction of wheel pressure is down on the part, toward the magnetic chuck which tends to hold the part tight in relation to the cutting action of the wheel. Entry and exit angles are not critical.
Direction of Cut
Milling says : Keep the castings in compression while machining. Although iron castings are relatively low in tensile strength, they possess relatively high compressive strength. Take advantage of this characteristic wherever possible by machining from the outside in, so that tool forces are directed toward solid metal in the casting. Finishing the cut in solid metal also avoids the tendency to pull metal.
Grinding says : In abrasive machining, no compromise needs to be made to favor the design of the piece part by cutting in any particular direction. In comparison with milling, the horizontal forces of grinding are much less, though the downward pressure may be as great or greater. So, while it makes sense to support any projections from the piece- part, where the design of the part does not provide adequate strength, there is no need to be concerned about the direction of the cut.
Clamping and Holding
Milling says : Select compatible clamping and holding methods. This is particularly important when machining malleable and Nodular iron castings, because they may deflect when force is applied. When a casting is machined in a deflected condition, it could be dimensionally out of tolerance when it is clamped. Be sure you support the casting well. This may require a special fixture, such as a pot-type retainer or one with spring-load snap-over clamps.
Grinding says : The magnetic chuck on a surface grinder is compatible with practically any design of iron casting, and the extra fixturing to accommodate odd geometry is usually simple. Castings can be roughed with maximum holding power on the magnetic chucks. After roughing the magnetic power can be reduced sparkout, any deflection can be relieved from a distorted part. Force sufficient to distort a part is unusual. Often some simple blocking is sufficient for support.
Milling says : Guard against high-frequency vibration. As has already been mentioned, a potential problem in machining thin- walled castings is chatter. Chatter at the tool cutting edge usually results in irregular machining of the workpiece and possible tool failure. Where a thin-wall section is necessary, the tool engineer may compensate by filling the casting with some liquid and allowing it to solidify before machining. Another technique is to use a spring- load damping device. A third alternative is the use of positive rake tools with light cuts.
Grinding says : In abrasive machining, vibration is a problem, as it is in any other machining operation. We also try to contain vibration, which helps to deteriorate the cutting action of the abrasive and increases the rate of wear. Chatter is less likely to occur on a rigid machine with light downfeed. Filling the casting with a material that solidifies at operating temperatures and melts at a slightly higher temperature is often practical, when the section cannot be ground otherwise.
In summary, compared with the single-point cutting tool, the grinding wheel:
1. Is less sensitive to hardness variations in the work material.
2. Is more economical in material cost, since it requires less stock machining.
3. Is not affected by interrupted cuts. In fact, performs better because of them.
4. Adjust better to thin-walled sections.
5. Is not affected by either draft in castings or tool entry or exit angles.
6. Requires less complicated workholding devices.